<form id="add_user_form" name="add_user_form">
    <fieldset>
        <legend contenteditable="false">用户信息</legend>
        <label contenteditable="false">用户序号</label>
        <input placeholder="学号、工号" type="text" id="username" name="username"><br>
        <label contenteditable="false">用户名</label>
        <input placeholder="真实姓名" type="text" id="name" name="name"><br>
        <label contenteditable="false">用户权限</label>
        <select name="level" id="level">
            <option value="1">学生</option>
            <option value="2">教师</option>
            <option value="3">管理员</option>
        </select><br>
        <button class="btn" onclick="addUser()" contenteditable="false" type="button">提交</button>
    </fieldset>
</form>